In the case of writing with a mechanical pencil, it is generally often the case that the mechanical pencil is not used in a situation where a body cylinder is perpendicular to a writing side (page), but used in a situation where the body cylinder is somewhat inclined to the writing side. In the case where the body cylinder is thus inclined for writing, there arises a phenomenon that a drawn line becomes bold as compared with that in the beginning, since the writing lead may locally abrade as the writing proceeds. Further, not only the drawn line changes in boldness, but also there arises a phenomenon that the drawn line changes in thickness (drawn line becomes thin) as the writing proceeds, since a contact area of the writing lead changes with respect to the writing side.
In order to avoid the above-mentioned problem, when the writing is carried out with the body cylinder being rotated, then it is possible to avoid such a problem that, as described above, the drawn line becomes bold as it is drawn, since a sharper side of the writing lead is rotatably in contact with the page when writing. However, when you write down with the body cylinder being rotated, there arises a problem in that operation of re-holding the body cylinder is required while the writing proceeds, leading to considerable reduction in writing efficiency.
In that case, it is not impossible to write down by re-holding the body cylinder and rotating it in a stepwise manner, in the case where exterior of the body cylinder is formed to be cylindrical. However, in the case of the mechanical pencil whose exterior may not be cylindrical and which may be designed to have a projection in the middle or which is a side-knock-type mechanical pencil, it is difficult to write by re-holding the body cylinder to be rotated in a stepwise manner as described above.
In order to solve such a problem, as described above, patent documents 1 through 3 disclose a mechanical pencil arranged such that a chuck for gripping a writing lead may be retreated by writing pressure, and having a rotational drive mechanism in which the above-mentioned writing lead together with the above-mentioned chuck is rotated by way of the retreat operation.
Patent Document 1: Japanese Patent No. 3882272
Patent Document 2: Japanese Patent No. 3885315
Patent Document 3: Japanese Utility Model Application Publication (KOKAI) No. S54-25339